B0 shimming device for mri

ABSTRACT

A magnetic resonance (MR) apparatus comprises magnet means for generating a main magnetic field in a sample region, encoding means for generating encoding magnetic fields superimposed to the main magnetic field, RF transmitter means for generating MR radiofrequency fields, driver means for operating said encoding means and RF transmitter means to generate superimposed time dependent encoding fields and radiofrequency fields according to an MR sequence for forming images or spectra; and acquisition means for acquiring an MR signal from said object. The magnet means comprise a primary magnetic field source providing a static magnetic field B 0  and at least one secondary magnetic field source providing an adjustable magnetic field B′. To provide improved shimming, the secondary magnetic field source comprises at least two spatially distinct portions of a first magnetic material and of a second magnetic material, respectively, said first magnetic material having a first magnetic moment density m 1  and said second magnetic material having a second magnetic moment density m 2 , and means for independently adjusting said second magnetic moment density m 2  by variation of an external control parameter.

FIELD OF THE INVENTION

The present invention generally relates to a magnetic resonance (MR)apparatus and a method of operating the same. More particularly, theinvention relates to the task of applying shimming corrections to themain magnetic field in a MR system, particularly an MR imaging (MRI)system.

BACKGROUND OF THE INVENTION

Susceptibility induced off-resonances challenge many cutting-edgemagnetic resonance applications using single shot read-outs, balancedacquisitions or high-resolution spectroscopy, in particular atultra-high fields.

Typically MRI and NMR magnets are constructed to deliver netuniformities in the region of the volume of interest of the order of 1ppm. The primary magnet itself, which is either wound by resistive orsuperconductive wires or constructed from a permanent magnetic material,does usually not deliver the required uniformity. Therefore correctionunits are installed. Most frequently ferromagnetic plates (shims) aredistributed along the bore such that the magnetic field becomes uniform[1, 2]. This so called “shimming” procedure is typically performed atproduction and/or installation of the magnet.

However, once the subject is exposed to the magnetic field of the mainmagnet, the susceptible material of the subject itself distorts themagnetic field in the subject. These distortions are usually measured atthe beginning of a sequence and compensated by actuating a set ofelectromagnets typically contracted from resistive coils. These shimcoils are typically designed to produce fields of spherically orcylindrically harmonic fields. The shim coils are either integrated inthe gradient coils, or dedicated insert shim coils lining the gradientcoil are used.

B₀ shimming with increased number of channels yields criticalimprovement of the achieved field uniformity. However, providing highnumbers of shim coils represents a trade-off with other specificationsof the MRI scanner. Integration of shim coils in the gradient tube canreduce the free bore space by thickening the gradient tube. Furthermore,additional heat is created in the gradient tube for which either thecooling has to be improved or the limits of the gradients have to bereduced. Furthermore, the conductors of the shim coils can carry eddycurrents induced by the fast gradient switching and thereby distort anddelay the gradient field. Furthermore, there can be substantive couplingto the gradient channels with the same adverse effect.

Using insert shim coils represents a similar trade-off; however the freebore diameter can be obtained back when unmounting the coil, but thisadditional handling effort is often a problem in practice due to thehigh weight and the cabling.

The weight and cabling size can be reduced by reducing the powerrequirements of the shim unit. Coil conductors in close proximity to thesubject [3] or even on the RF coils [4] are significantly moreefficient. Thereby the size of the coil conductor windings and the powerrequirements are drastically reduced. However, unwanted interactionswith the RF operation of the scanner are hard to tame and also couplingto the switching gradients is an obvious problem. Also, the handling isfurther aggravated by the amount of conductors in the unit and the largenumber of high-current wires routed through the bore. Still further,highly stabilized current supplies need to be fitted into the technicalroom.

US 2006/113995 A1 discloses a magnetic field generating apparatus and amethod for magnetic resonance imaging which comprises two spatiallydistinct sets of shim elements and means for independently regulatingthe temperature of at least a portion of one set of shim elements inorder to compensate for B0 field variations caused by a temperaturedrift in the apparatus. Similar arrangements are also described in JPH08 215168 A, US 2010/045293 A1 and US 2010/207630 A1, which all aimeither at compensating the effect of an overall temperature drift or atreducing magnetic field inhomogeneity present after initial set-up of anMRI arrangement.

SUMMARY OF THE INVENTION

In view of the above there is a need for an improved local B₀ shimmingthat overcomes or at least reduces the above mentioned disadvantages ofelectromagnetic shim elements and which allows for reduction of magneticfield distortions caused by the introduction of a subject or object intothe MR apparatus.

Therefore, according to one aspect of the invention, there is provided amagnetic resonance (MR) apparatus comprising:

-   a) magnet means for generating a main magnetic field in a sample    region;-   b) encoding means for generating encoding magnetic fields    superimposed to the main magnetic field,-   c) RF transmitter means for generating MR radiofrequency fields;-   d) driver means for operating said encoding means and RF transmitter    means to generate superimposed time dependent encoding fields and    radiofrequency fields according to an MR sequence for forming images    or spectra; and-   e) acquisition means for acquiring an MR signal from said object;

wherein said magnet means comprise a primary magnetic field sourceproviding a static magnetic field B₀ and at least one secondary magneticfield source providing an adjustable magnetic field B′;

wherein said secondary magnetic field source comprises at least twospatially distinct portions of a first magnetic material and of a secondmagnetic material, respectively, said first magnetic material having afirst magnetic moment density m1 and said second magnetic materialhaving a second magnetic moment density m2, and means for independentlyadjusting said second magnetic moment density m2 by variation of anexternal control parameter, the apparatus further comprising means fordetermining an instant spatial distribution of the static magnetic fieldresulting from said static magnetic field B0 and said adjustablemagnetic field a.

In the present context, denoting different magnetic materials as “firstmagnetic material” and “second magnetic material” will follow theconvention that the material used for adjusting the magnetic field B′will be denoted as “second magnetic material”. This does not rule outthat the other (i.e. the “first”) magnetic material could also have anadjustable magnetic moment density, but this is not mandatory in thecontext of the present invention. It would also be possible to use anarrangement wherein the first magnetic material has a vanishingly smallmagnetic moment density m1.

The secondary magnetic field sources according to the present inventioncorrespond to what is often called “shim units” in the field of MRtechnology.

Another aspect of the invention relates to a method of operating an MRapparatus as defined above, wherein said external control parameter isadjusted in such manner as to obtain a predetermined main magnetic fieldB resulting from the superposition of the static magnetic field B₀ andthe adjustable magnetic field B′ of each secondary magnetic fieldsource, the adjustment being carried out at the beginning of an MRsequence for forming images or spectra, after a sample or subject hasbeen introduced into the sample region.

Advantageous embodiments are defined in the dependent claims anddescribed further below.

According to a particularly advantageous embodiment (claims 2 and 12),the second magnetic moment density m2 is adjustable in a range extendingfrom values that are smaller than the first magnetic moment density m1to values that are larger than the first magnetic moment density m1.

In principle, any external influence factor that allows for efficientand reliable adjustment of the magnetic moment density could be used asexternal control parameter in the sense of the present invention.Advantageously, the external control parameter is selected from thegroup consisting of temperature, pressure, shear and light illuminationprevailing at the secondary magnetic field source and electric currentflowing therethrough (claim 3).

According to a particularly advantageous embodiment (claim 4), theexternal control parameter is the local temperature of the portion ofsecond magnetic material and the adjusting means are configured toadjust said local temperature in a control temperature range extendingfrom a lowest control temperature T_(L) to a highest control temperatureT_(H), the second magnetic material having a Curie temperature T_(C)lying within said control temperature range. As generally known, whenferromagnetic materials are heated above their Curie temperature theyexperience a marked loss in their magnetic moment density and undergo atransition from ferromagnetic to paramagnetic behavior. This is mostadvantageous in combination with the embodiment of claim 2, because itallows providing a direction-switchable secondary magnetic field sourcewithout having to use diamagnetic materials. Diamagnetic materialsgenerally have a magnetic susceptibility that is only weakly dependenton temperature and thus are not well suited for building up anadjustable magnetic field source.

As generally known, the Curie temperature is a material-dependentproperty that varies over a wide range. It is also known that e.g. incertain alloys it is possible to adjust the Curie temperature over aconsiderable range by variation of the composition. For many instanceswhere the present invention may be useful, it will be advantageous touse compounds with a Curie temperature that is somewhat higher thanambient temperature, meaning that one could switch between ferromagneticand paramagnetic behavior by raising the temperature by a certain amountabove ambient temperature. Therefore, according to one embodiment (claim5) the Curie temperature T_(C) of the second magnetic material is in therange from 300K to 450K. For example, one can use a nickel-copper alloywith a composition Ni75Cu25, which has a Curie temperature of about 80°C.

According to a further embodiment (claim 6), the secondary magneticfield source is formed as an elongated body having a longitudinal axiswherein a plurality of spatially distinct portions are arranged atdistinct positions along said longitudinal axis. For example, eachspatially distinct portion can be electrically heated by driving acurrent through a resistor. Its temperature is locally measured via athermocouple. Each of these units is thermally isolated and RF shielded.Alternatively, optical fibers can be employed to deliver heat to themagnetic material and reading out the temperature (i.e.fluoro-optically). This offers the advantage of very good decoupling ofthe shim units from the RF and the gradient switching in the scanner,albeit at a higher complexity. Still alternatively, Peltier or anequivalent type of elements allowing heating and cooling can beemployed.

For many applications, e.g. for whole body MRI scanners, it will beadvantageous to have an embodiment comprising a plurality of secondarymagnetic field sources arranged circumferentially around the sampleregion (claim 7). Such an arrangement provides for good compensation ofany unwanted distortions of the main magnetic field in the region ofinterest.

According to a further embodiment, each secondary magnetic field sourcecomprises means for determining an instant value of the external controlparameter (claim 8). This allows for continuous and rapid monitoring andadjustment of the operation of the secondary magnetic field sources.When using local temperature as the external control parameter, thedetermining means can be selected from known temperature sensor typessuch as e.g. thermistors or thermocouples. The temperature isadvantageously controlled via a close loop controlling system. Thecontroller can be positioned in the bore of the MR system, e.g. an MRIsystem. The controller consists of a small multichannel PWM currentlow-side driver, multichannel thermocouple read-out units and amicrocontroller. The microcontroller has a data link out of the bore.The microcontroller reads the data obtained from the thermocouple andadjusts the PWM ratio of the current source. A PID controller can beused for this purpose.

According to the present invention it is contemplated that the materialsto be used as first and/or second magnetic materials could be selectedfrom a large variety of magnetic materials. According to an advantageousembodiment (claim 9), the second magnetic material is formed of an alloycontaining Fe, Ni, Ga, Mn, Gd, Sm, Nd, Dy, Eu or Co.

In an advantageous embodiment (claim 10), the adjustable magnetic fieldB′ of each secondary magnetic field source in an axial direction z ofthe static magnetic field B₀ can be switched between positive andnegative values as a function of the external control parameter. This isparticularly useful for achieving an appropriate shimming action over awide adjustment range.

As evident from the above, we hereby disclose a novel approach to localB₀ shimming using materials whose magnetic moment arising from thematerial's magnetic susceptibility can be controlled in the bore of theMRI scanner. In this manner the advantages of the passive shimmingapproach (low power, very high order) can be combined with theadvantages of the active shim coils of being adaptive to the sample'ssusceptibility distribution.

The magnetic moment of the material is hereby controlled by controllingthe temperature, the pressure/shear [5], light illumination [6] and/orthe electric current and or field through a portion of magneticmaterial.

In a particular embodiment, the magnetization of a portion of themagnetic material is controlled by a heater. The heat is applied by anelectric current through a resistor or resistor wire that is regulatedusing a temperature sensor measuring the temperature of said portion ofmagnetic material. Thereby the temperature and concomitantly themagnetic moment of the material can be controlled in a closed loop. Theelectric current can be provided by a DC, AC or a modulated source.

The temperature dependence of the material is thereby a design parameterin the system. A high Curie point gives a large range for control whilea Curie point close to room temperature is in particular energyefficient. The Curie point of materials can be controlled via thestoichiometry of the alloy, e.g. [7].

The adaptive shimming will be achieved by the field produced by themagnetic material. However, typically net shimming fields with bothsigns are required, but materials with susceptibilities that can bealtered with both signs are typically not available. However, thecontrollable materials can be arranged such that the magnetic fieldproduced by the arrangement outside the magnetic material can have bothsigns dependent on the magnetism of one portion of the material. Therebyboth polarities of shim fields can be generated. A particular example isa substantially cylindrical, arrangement with its axis along theexternal magnetic field that is longer than the volume of interest. Ifthe magnetic moment density of the arrangement is constant, the fieldinduced by the arrangement outside the cylinder is substantially zero.Locally increasing the magnetic moment density of portion of thearrangement produces an additional net field outside the cylinder ofsaid increase in magnetic moment, conversely a reduction produces afield with opposite sign.

The energy efficiency of the approach is furthermore determined by theprovided thermal isolation of the controlled magnetic material portion.Materials with very low thermal conductivity such as foams, glass,vacuum capsules or aerogels can be employed. Furthermore materials withhigh thermal conductivity but preferentially low electrical conductivitycan be employed for thermal grounding of the structure i.e. avoidingspreading of the heat and reducing heat transfer through the thermalisolation by flattening hot spots. Such materials are typically veryhard crystals or ceramics such as aluminum oxide, aluminum nitrate.

The power efficiency is, given a high thermal isolation, much lower thanwith resistive shim coils. Furthermore, coupling to the switchinggradient fields and field distortions induced by eddy currents runningon the shim units are intrinsically very low due to the small size ofthe shimming units.

The speed at which the shimming fields can be ramped depends on thesteepness of temperature dependence of the magnetism, the thermalcapacity of the shim unit and the heating or cooling power that can besupplied. The thermal capacity can be kept low by choosing materialswith high magnetization mass density. For speeding up the heatconduction in the unit a magnetic material with high heat conduction canbe chosen. Furthermore, the magnetic material can be structured (by thinlayering or cladding) with a material with high heat conduction in orderto accelerate the heat transport in the magnetic material. The thermalcapacity is optimized by choosing a material. Therefore micromachining,plating or sputtering of magnetic materials on highly heat conductivematerials, or vice-versa can be beneficial.

Furthermore heat pumps such as Peltier elements can be employed in orderto heat and cool the material. This would not only speed up the slewingof the magnetic shimming fields but would also allow using materialswith Curie temperatures close to room temperature improving the energyefficiency maintaining high slew rates.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other features and objects of this invention andthe manner of achieving them will become more apparent and thisinvention itself will be better understood by reference to the followingdescription of embodiments of this invention taken in conjunction withthe accompanying drawings, wherein are shown:

FIG. 1 A schematic representation of one embodiment of a secondarymagnetic field source providing an adjustable magnetic field.

FIG. 2 A schematic representation of a further embodiment of a secondarymagnetic field source providing an adjustable magnetic field.

FIG. 3 An exemplary arrangement of an MRI apparatus including aplurality of adjustable magnetic field elements.

FIG. 4 a) Typical temperature dependence of the magnetic moment density(m) of a ferromagnetic particle in a strong external field. Towards theCurie temperature (T_(C)) the strong ferromagnetic moment diminishes.Above T_(C) the paramagnetism scales still down with temperature. b)T_(C) of cupric alloys can be adjusted by the admixture of copper.Thereby the temperature range in which a magnetic particle has to becontrolled can be adjusted.

FIG. 5 Spatial distribution of magnetic material. The distribution ofthe static material (m) and the controllable units (m_(c)(T)) settled athalf of their maximum moment provides zero magnetic field outside thecylindrical structure given it is much longer than the volume ofinterest. Lowering one unit's temperature provides then a net field of aparamagnet and increasing the temperature that of a net strongdiamagnet. The shim units can thereby adjust the fields with both signs(bipolarly).

FIG. 6 Setup. The controllable units (a) were made of a (3 mm)³ Ni75Cu25shot thermally coupled to SMD resistors. A Pt500 measures itstemperature. 50 μm diameter wires are used for connecting. A wrapper offoam is applied for isolation and aluminum foil for RF shielding. b)Photograph of a unit. c) setup for the measurements with 3 controllableunits. The static material distribution has been discretized into shotsof half the volume than the units because no wire material wasavailable.

FIG. 7 a) Field as recorded by the field probes heating m1-m3sequentially. b) B₀ field maps in s1-s4 acquired as marked in a).Isolines at 0±15±30±50±100±200 Hz. The fields show a distinct dipolarpattern in the range of ±300 Hz. Close range ripples (solid arrow)result from the discretization of the magnetization distribution intoshots. Casting the material into cylinders should remove this problem.

c) off-resonance line profiles 4 cm distant from the shim units.

DETAILED DESCRIPTION OF THE INVENTION

In the following we will use temperature T as a representative exampleof an external control parameter that influences the magnetic momentdensity of a material of interest. As explained earlier, there are otherquantities that could be used as control parameter instead oftemperature, so the following examples shall not be construed as alimitation to the use of temperature as the control parameter.

The basic principle of an adjustable secondary magnetic field source isillustrated in FIG. 1. The device shown in FIG. 1a comprises a rod-likeobject formed predominantly of a first material with a substantiallyuniform magnetic moment density m1=m and having a central section formedof a second material with a magnetic moment density m2(T) that istemperature dependent. The device is placed in an external magneticfield B₀ oriented in z-direction.

FIG. 1b shows the situation where the temperature of the central sectionhas been set to establish a local magnetic moment density that is higherthan that of the surrounding parts of the rod, i.e. m2(T)>m. Thissituation corresponds to an arrangement wherein one magnetic dipole islocated in the central part of the rod-like object. In contrast, FIG. 1cshows the situation where the temperature of the central section hasbeen set to establish a local magnetic moment density that is lower thanthat of the surrounding parts of the rod, i.e. m2(T)<m. This situationcorresponds to having a pair of magnetic dipoles located at the two endsof the rod-like object with a gap in the central part having a magneticfield distribution corresponding to an arrangement with one centrallyarranged magnetic dipole now pointing in the opposite direction ascompared to FIG. 1b . It should be noted that in the case of a longrod-like object the two terminal magnetic dipoles shown in FIG. 1c wouldbe longitudinally stretched and thus the resulting field surrounding theobject would be substantially zero. This would effectively lead to asituation as shown in FIG. 1 d.

If the second material exhibits ferromagnetic behavior, the highmagnetic moment situation shown in FIG. 1b can be established by keepingthe local temperature T below the Curie temperature T_(C) whereas thelow magnetic moment situation shown in FIG. 1c can be established bykeeping the local temperature T above the Curie temperature T_(c).

From a comparison of FIG. 1b on the one hand and FIGS. 1c and 1d on theother hand, it is seen that the secondary magnetic field in the regionsurrounding the central section has opposite directions in the twocases.

A more complex situation is shown schematically in FIG. 2. Here arod-like device comprises an elongated element formed of a firstmagnetic material having a first material M1 with a substantiallyuniform magnetic moment density m1=m and two portions of a secondmagnetic material M2 arranged axially displaced from each other at axialpositions z_(a) and z_(b), respectively. In the situation shown in FIG.2, the portions shown at the left and right, respectively, are set todifferent temperatures T_(low) and T_(high), respectively. Inparticular, T_(low) and T_(high) can be adjusted to be below and abovethe Curie temperature T_(C), respectively. With such an arrangement itis possible to generate secondary magnetic fields with disparate localpatterns. In the example shown, it is indicated schematically how thesecondary magnetic field in the neighborhood of the portion at z_(a) hasa component B_(z)(z_(a)) directed in positive z-direction, whereas inthe neighborhood of the portion at z_(a) the secondary magnetic fieldhas a component B_(z)(z_(b)) directed in negative z-direction.

An exemplary arrangement of an MRI apparatus including a plurality ofadjustable magnetic field elements acting as secondary magnetic fieldsources for providing an adjustable magnetic field B′ is shown in FIG.3. The arrangement comprises a radiofrequency (RF) coil 1 and acylindrically formed element 2 made of a first magnetic material M1 andprovided with a plurality of distinct shimming portions 3 of a secondmagnetic material M2. The element 2 is arranged surrounding aschematically shown sample or object zone 4. As generally known for sucharrangements, the RF coil 1 is provided with a plurality of capacitorelements 5.

Example 1: Magnetic Pebbles—Materials with Controllable Magnetism forCompact, Low-Power Shim Units Introduction

Susceptibility induced off-resonances challenge many cutting-edgeapplications using single shot read-outs, balanced acquisitions orhigh-resolution spectroscopy, in particular at ultra-high fields. B₀shimming with increased number of channels yields critical improvements[8], but significantly reduces free bore diameter. Coil conductors inclose proximity to the subject [9] or even on the RF coils [4] reducespace and power requirements. However, unwanted interactions with RF andgradient operation was reported as a main issue and the handling isaggravated by the amount of conductors in the unit and the large numberof high-current wires routed through the bore. Furthermore highlystabilized current supplies need to be fitted in the technical room.

As an alternative we present the integration of ferromagnetic materialswhose magnetization can be accurately controlled in-situ as opposed totraditional passive shims [1]. Thereby the secondary field produced bythis material is used to shim highly localized. The proposed geometricarrangement allows producing fields with both polarities.

Methods

The magnetic moment density (m) of a ferromagnetic particle in a strongexternal magnetic field can be controlled by its temperature ([10], FIG.4a ). At Curie temperature (T_(C)) the ferromagnetism vanishes andrenders the material paramagnetic. By heating its magnetic moment isgradually reduced by more than an order of magnitude, correspondinglythe secondary fields nearly vanishes. Hence by controlling the materialtemperature a shim field can be tailored.

For obtaining a reasonable temperature range, Nickel and Copper werealloyed to shots in a Ni75Cu25 stoichiometry [7] resulting in a T_(C) of350° K (FIG. 4b ). Each shot of ˜(3 mm)³ is restively heated, itstemperature is measured by a Pt500 thermistor and it is thermallyisolated and shielded (FIGS. 6a and b ).

To produce fields of both polarities using materials with only positivesusceptibilities, the magnetic material is arranged in a matrix suchthat its net secondary field is uniform when the magnetization of theheated particles is roughly halved (e.g. FIG. 5). In a first example, 35particles were arranged on the axis of the main magnetic field by pressfitting in a wooden bar with 16 mm distance (FIG. 6c ). Threecontrollable magnetic particles (having 2 m at low temperatures) weremounted in the center-slots, obtaining from each a net field outside thecylinder of approximately a dipole with a magnetic moment of −m to +m.

The secondary field of the shim unit was measured by B₀ mapping (Philips3T Achieva, Best Netherlands) with 3 ms echo-spacing in a phantom bottleplaced directly on top of the unit. 3 magnetic field probes (Skope MRT,Zurich, Switzerland) were each placed about 1 cm from each unit and 2 ontop of the bottle.

Results

FIGS. 7a and 7b show the net B₀ fields induced by the three shim unitstemporally and spatially. Voltages from 0-10 V were applied in stepsresulting in a temperature range from 293-420° K. The surface of theunit did warm up hardly noticeably. The slew rate was about 1 μT/s. FIG.7b shows field profiles in about 4 cm distance from the units. Theripples in the shim field next to the unit in the sagittal images resultfrom the discretization of the magnetic moment distribution from acontinuous cylinder into individual shots. They are expected to bedrastically reduced once the NiCu is cast into a cylindrical geometry.

Discussion

Particles with controllable magnetism produce shim-field patterns withhigh spatial degrees of freedom. Since the source of the field is not anelectric current but the magnetism of the material, smaller form factorsand lower current consumptions are achieved and the particles are welldecoupled from gradient and shim as well as from RF coils. Opposed topassive shims, rearranging the material is not required to fit subjectspecific susceptibility distributions.

The heat required to control the units can be administered by DC and ACcurrents as well as optically tunable materials can be employed [11].Furthermore the power delivery for the heating can be efficientlymodulated by switched mode schemes such as by PWM current sourcessimilarly as used for LED lightings where tens of channels can be housedin a single IC package. This allows placing the required electronics inthe bore which dramatically reduces the involved cabling efforts.

Very large numbers of independent shim channels can be integrated in RFcoils with low additional weight and space requirements. Thereby highdegrees of freedom can be obtained therefore the approach is expected tobe well suited for shimming of susceptibility induced off-resonancesi.e. in the prefrontal cortex, ear channels or the spine.

Example 2: Adaptive Shimming Procedure Preparations

The dependence and spatial pattern of the fields induced by theindividual shim units in the volume of interest in the subject have tobe known in advance. There are various methods for obtaining thisinformation. Typically this information will be gathered during thedesign or installation/maintenance procedure of the device.

The field patterns can be either obtained by magnetostatic fieldsimulations/calculations using the knowledge of the geometricaldistribution of the employed materials and its magnetic properties.Alternatively, the fields can be measured using a field camera, ascanning magnetic field probe or an MRI based tomographic procedure (B0mapping sequence, [12, 13]) performed on a phantom or in-vivo/in-situ.For isolating the fields induced by the shimming units from the fieldinduced by other structures and magnets, the field can be measured withdifferent control parameter values or with and without shimming unitpresent and the measurements can subsequently be compared.

Similarly, the dependence of the induced shimming field on the controlparameter can be directly measured in an MRI scanner using one of themethod mentioned above. Thereby it might be sufficient in many cases toacquire a calibration curve for the employed material and using thisinformation together with the spatial distribution of the material tocalculate an estimate of the induced fields.

Furthermore, in most cases the spatial distribution of the inducedfields is linearly dependent on the magnetic moment of the employedmaterial at the given control variable. Therefore theacquisition/estimate of the field distribution can be separated from theestimation of the magnetic moment of the material. Consequently, onlyfew field maps covering the entire volume are required.

Finally, since the induced correction fields are in most cases verysmall compared to the background fields, the individual shim units willonly interact very weakly. Consequently, the induced total fields of anarray of such shim units will be linearly dependent on the induced fieldof each unit alone. This linear relation significantly simplifies thecalibration procedure in that each unit can be measured separately andaccounted according to its geometric position relative to the volume ofinterest. Furthermore, the calculation of the induced total field isgreatly simplified too, which makes the subsequently describedoptimization procedures much simpler.

Procedure

Before starting the intended MR signal acquisition, the B0 field in thevolume of interest is measured in-situ. I.e. the subject/sample ispositioned in the scanner as suitable for the subsequent scanningprocedure. Well known measurement procedures for active shimming can beemployed [13-16] by which the field is obtained on full grid or onprojections to appropriate basis functions.

If the shimming units are mounted on movable parts such as a local RFcoil or the patient support, the position of the shim units or at leastthe entire array has to be determined. This can be achieved either byoptical markers for positioning or referencing to light visors, directmechanical or optical measurements or by acquiring and evaluating thesignal from NMR active fiducial markers or field probes duringpositioning scans.

Using the knowledge of the field induced by each shim unit in thesample, the control value for each shim unit can be calculated such thatthe field distribution in the volume of interest will approximate thegiven target distribution (typically a uniform field) best. For thispurpose, well-known optimization techniques can be employed. In mostcases it will be beneficial to employ constrained optimizationalgorithms incorporating the maximum fields that can be induced by eachshimming unit.

The calculated control values are then applied to the shim units.

The procedure described above can then be iterated in order to optimizethe accuracy of the field correction. In other words, once thecalculated control values have been applied to the shim units, one canrepeat the in situ measurement of B0 in the volume of interest and ifthe deviation from the target distribution is larger than a pre-definedthreshold, one can calculate and apply a refined set of control values.

Once the B0 field in the volume of interest is deemed acceptable, onecan proceed to carry out the intended MRI signal acquisition.

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1. A magnetic resonance (MR) apparatus comprising: a) magnet means forgenerating a main magnetic field in a sample region; b) encoding meansfor generating encoding magnetic fields superimposed to the mainmagnetic field, c) RF transmitter means for generating MR radiofrequencyfields; d) driver means for operating said encoding means and RFtransmitter means to generate superimposed time dependent encodingfields and radiofrequency fields according to an MR sequence for formingimages or spectra; and e) acquisition means for acquiring an MR signalfrom said object; wherein said magnet means comprise a primary magneticfield source providing a static magnetic field B₀ and at least onesecondary magnetic field source providing an adjustable magnetic fieldB′; wherein said secondary magnetic field source comprises at least twospatially distinct portions of a first magnetic material and of a secondmagnetic material, respectively, said first magnetic material having afirst magnetic moment density m1 and said second magnetic materialhaving a second magnetic moment density m2, and means for independentlyadjusting said second magnetic moment density m2 by variation of anexternal control parameter, the apparatus further comprising means fordetermining an instant spatial distribution of the static magnetic fieldresulting from said static magnetic field B0 and said adjustablemagnetic field B′.
 2. The MR apparatus according to claim 1, whereinsaid second magnetic moment density m2 is adjustable in a rangeextending from values that are smaller than the first magnetic momentdensity m1 to values that are larger than the first magnetic momentdensity m1.
 3. The MR apparatus according to claim 1, wherein saidexternal control parameter is selected from the group consisting oftemperature, pressure, shear and light illumination prevailing at thesecondary magnetic field source and electric current flowingtherethrough.
 4. The MR apparatus according to claim 3, wherein saidexternal control parameter is the local temperature of said portion ofsecond magnetic material, and wherein said adjusting means areconfigured to adjust said local temperature in a control temperaturerange extending from a lowest control temperature T_(L) to a highestcontrol temperature T_(H), and wherein said second magnetic material hasa Curie temperature T_(C) lying within said control temperature range.5. The MR apparatus according to claim 3, wherein said Curie temperatureT_(C) is in the range from 300K to 450K.
 6. The MR apparatus accordingto claim 1, wherein said secondary magnetic field source is formed as anelongated body having a longitudinal axis, and wherein a plurality ofspatially distinct portions of said second magnetic material arearranged at distinct positions along said longitudinal axis.
 7. The MRapparatus according to claim 6, comprising a plurality of secondarymagnetic field sources arranged circumferentially around the sampleregion.
 8. The MR apparatus according to claim 1, wherein each secondarymagnetic field source comprises means for determining an instant valueof the external control parameter.
 9. The MR apparatus according toclaim 1, wherein said second magnetic material is formed of an alloycontaining Fe, Ni, Ga, Mn, Gd, Sm, Nd, Dy, Eu or Co.
 10. The MRapparatus according to claim 1, wherein the adjustable magnetic field B′of each secondary magnetic field source in an axial direction z of thestatic magnetic field B₀ can be switched between positive and negativevalues as a function of the external control parameter.
 11. A method ofoperating the MR apparatus according to claim 1, wherein said externalcontrol parameter is adjusted in such manner as to obtain apredetermined main magnetic field B resulting from the superposition ofthe static magnetic field B₀ and the adjustable magnetic field B′ ofeach secondary magnetic field source, wherein the adjustment is carriedout at the beginning of an MR sequence for forming images or spectra,after a sample or subject has been introduced into the sample region.12. The method according to claim 11, wherein said second magneticmoment density m2 is adjustable in a range extending from values thatare smaller than the first magnetic moment density m1 to values that arelarger than the first magnetic moment density m1.
 13. The MR apparatusaccording to claim 3, wherein said secondary magnetic field source isformed as an elongated body having a longitudinal axis, and wherein aplurality of spatially distinct portions of said second magneticmaterial are arranged at distinct positions along said longitudinalaxis.
 14. The MR apparatus according to claim 13, wherein said externalcontrol parameter is the local temperature of said portion of secondmagnetic material, and wherein said adjusting means are configured toadjust said local temperature in a control temperature range extendingfrom a lowest control temperature T_(L) to a highest control temperatureT_(H), and wherein said second magnetic material has a Curie temperatureT_(C) lying within said control temperature range.
 15. The MR apparatusaccording to claim 13, wherein said Curie temperature T_(C) is in therange from 300K to 450K.
 16. The MR apparatus according to claim 13,wherein said secondary magnetic field source is formed as an elongatedbody having a longitudinal axis, and wherein a plurality of spatiallydistinct portions of said second magnetic material are arranged atdistinct positions along said longitudinal axis.
 17. The MR apparatusaccording to claim 16, comprising a plurality of secondary magneticfield sources arranged circumferentially around the sample region.
 18. Amethod of operating the MR apparatus according to claim 3, wherein saidexternal control parameter is adjusted in such manner as to obtain apredetermined main magnetic field B resulting from the superposition ofthe static magnetic field B₀ and the adjustable magnetic field B′ ofeach secondary magnetic field source, wherein the adjustment is carriedout at the beginning of an MR sequence for forming images or spectra,after a sample or subject has been introduced into the sample region.19. The method according to claim 18, wherein said second magneticmoment density m2 is adjustable in a range extending from values thatare smaller than the first magnetic moment density m1 to values that arelarger than the first magnetic moment density m1.